Defining JSON_MEMORY_MANAGE generates this error:
Source/libjson.cpp|409| error: invalid conversion from ‘void*’ to ‘char*’
To me, looks like you just need to cast to json_char* at the point of return.
btw, perhaps we should switch to static_cast<> casting rather than (c) casting. Its easier to see on the screen when its being used, and easier to grep for.